1. Field of the Invention
The present invention relates to a multilayer dressing for the care and treatment of wounds. In particular, the present invention relates to a multilayer dressing having therapeutic and prophylactic properties, and methods for making the dressing.
2. Discussion of Background
The antimicrobial and antifungal properties of silver and silver compounds are well known. Topical preparations that contain silver or silver compounds silver nitrate solution, silver sulfadiazine cream, colloidal silver compositions, silver-protein compounds such as Argyrol™, and so forth—are widely used in medicine. The useful effects of these compositions are due to the small amounts of free silver ions produced by dissociation of the silver compound or to formation of toxic by-products in situ.
The effectiveness of silver as an antimicrobial agent is at least partly determined by the delivery system. Most silver compounds that dissociate readily and produce large numbers of free silver ions are highly toxic to mammalian (including human) tissues. Less-toxic compounds, including silver sulfadiazine cream (widely used in the treatment of burns) and silver nitrate solution, do not dissociate readily and therefore do not release large numbers of silver ions. These compounds must be re-applied frequently to maintain their clinical efficacy.
Electrically-generated silver ions, which can penetrate more deeply into the tissues, are effective even against antibiotic-resistant strains of bacteria, fungi, etc., inhibiting growth in vivo and in vitro at current densities as low as 10 nA/mm2 and silver ion concentrations as low as 0.5 mg/ml. The effects of electrically-generated silver ions are described in a number of publications, including the following: J. A. Spadaro, et al., “Antibacterial Effects of Silver Electrodes with Weak Direct Current,” Antimicrobial Agents &Chemotherapy, Vol. 6, pp. 637–642 (1974); T. J. Berger, et al., “Antifungal Properties of Electrically Generated Metallic Ions,” Antimicrobial Agents &Chemotherapy, Vol. 10, pp. 856–860 (1976); R. O. Becker, et al., “Treatment of Orthopedic Infections With Electrically-Generated Silver Ions,” J. Bone &Joint Surgery, Vol. 60-A, pp. 871–881 (1978)).
Silver and other metals are widely used in wound dressings and materials therefor. Fabo (U.S. Pat. No. 5,340,363) discloses a dressing that includes an outer absorbent layer and an inner porous, hydrophobic layer knitted of elastic threads and encapsulated by a soft, hydrophobic silicone or polyurethane gel. The gel can be used as a carrier for antibacterial agents such as zinc, pain-relieving substances, and agents that stimulate wound repair. Klippel, et al. (U.S. Pat. No. 3,830,908) use micronized allantoin as a carrier for a bactericidal or bacteristatic ingredient (such as silver citro allantoinate) that is dispersed on the surface of a plastic air splint or other bandaging product. McKnight, et al. (U.S. Pat. No. 3,800,792) disclose a surgical dressing having a layer of tanned, reconstituted collagen foam film laminated to a thin, continuous layer of an inert polymer. The collagen layer contains finely-divided silver metal added by soaking the collagen film in Tollen's reagent. Stowasser (U.S. Pat. No. 2,934,066) makes a dressing of absorbent, metal-coated fibers, such as a carding fleece coated with aluminum and backed by compressed cellulose, and polyamide fibers coated with vacuum-deposited silver.
Dressings for provision of electrical stimulation are also known. For example, Jones (U.S. Pat. No. 4,911,688) covers a wound with a clear cover that serves as a hollow chamber for holding a fluid such as saline in contact with a wound. When connected to a voltage source, a metal anode and a return electrode create free ions and an electrical field to enhance healing and tissue regeneration. Juhasz (U.S. Pat. No. 4,817,594) discloses a multi-layer dressing for covering discharging, malodorous wounds. The dressing includes a layer of an electrically-conductive material such as silver and a layer of charcoal fabric. Application of a DC (direct current) voltage to the conductive layer drives silver ions into the wound to enhance tissue growth and inhibit bacterial growth; application of transcutaneous AC (alternating current) is used for post-operative pain relief Seiderman (U.S. Pat. No. 4,767,401) describes a bandage like device used for iontophoretic administration of medicaments, including silver-protein colloids. The device includes a metal foil electrode (preferably aluminum), and makes use of the slight inherent negative electric charge proximate a wound site to generate a small electric field at the site.
Matson (U.S. Pat. No. 4,728,323) coats a substrate (nylon fabric, polymeric film, fiberglass, gauze or polyurethane foam) with a film of a silver salt deposited by vapor or sputter coating techniques. Alternatively, fibers can be coated and then woven or knitted into a fabric. Konikoff (U.S. Pat. No. 4,142,521) shows a bandage or surgical sponge material incorporating one or more electret elements, each electret providing a small electrostatic field to the area of the wound.
In U.S. Pat. No. 5,814,094, Becker, et al. disclose a bimetallic fabric woven of nylon fibers coated with a first metal such as silver, interspaced at intervals with fibers coated with a second metal such as gold or platinum, preferably in a ratio of about 10:1. Alternatively, deposits of the second metal are placed on a fabric that contains the first metal. When contacted with an electrolyte, each contact junction between the first and second metals serves as a bimetallic junction that produces free silver ions. The material may be used in therapeutic or prophylactic treatment of wounds (including surgical incisions). An iontophoretic system for promoting tissue healing processes and inducing regeneration is described in application Ser. No. 08/623,046, filed Mar. 28, 1996. The system is implemented by placing a flexible, silver-containing anode in contact with the wound, placing a cathode on intact skin near the anode, and applying a wound-specific DC voltage between the anode and the cathode. Electrically-generated silver ions from the anode penetrate into the adjacent tissues and undergo a sequence of reactions leading to formation of a silver-collagen complex. This complex acts as a biological inducer to cause the formation in vivo of an adequate blastema to support regeneration. The disclosures of the above-referenced patent applications are incorporated herein by reference.
Regardless of whether silver is provided in the form of silver ions or as a topical composition (silver nitrate solution, silver sulfadiazine cream, etc.), its beneficial effects are manifested primarily at the treated surface and immediately adjacent tissues, and are limited by the achievable tissue concentration of silver ions. Despite the availability of numerous techniques for the delivery of silver and silver compounds in vitro and in vivo, there remains a need for a delivery system that is capable of supplying clinically useful concentrations of silver ions to a treatment site without the need for adjuvant electrical stimulation.